2. Field Of The Invention
This invention relates to transducers that convert sound, vibration, or other forms of varying energy into electrical signals for transmission, storage, display, or analysis, such as microphones, pick-ups, and other audio transducers.
2. Description of Prior Art
Transducers convert various forms of energy, such as light, sound, heat, motion, etc., from one form to another. Transducers used in the electronics industry include microphones, pick-ups, accelerometers, and other types of transducers, which are well known to those skilled in the art. In general, these provide some form of electrical analog output signal; the voltage, current, or some other quality of the signal, varies in accordance with the input to the transducer. Many of these transducers require preamplifiers, to boost the output signals's level; transform impedance, (for example, high impedance to low impedance); and/or to drive the cable that connects the transducer to the equipment that is receiving and utilizing the transducer's output signals.
A typical example of a transducer is a studio sound microphone, in which sound wave are converted into a very low level electrical signal by a microphone element, such as a magnetic-dynamic, electrostatic, or crystal element. The output of the microphone element is then applied to an impedance matching transformer, and/or a preamplifier, to convert the signal to a suitable impedance level and voltage for transmission. The transformed electrical signal is applied to a cable, which may be 100 feet or more in length. The far end of the cable enters another transformer, amplifier, or other interface device, and the output of this, in turn, is used by a studio console, tape recorders, signal analyzer, etc. to record or process the sound originally produced at the transducer.
The analog technique of transducing and transmitting audio signals has been in use for over 100 years, and is still in use, even in studios with modern digital tape recording. The fidelity of analog transmission is restricted by interference, such as noise, distortion, the dynamic range of analog systems, ambient magnetic and electrical fields, etc. The limited dynamic range (60-70 dB) and high (up to 3%) harmonic distortion of the analog recording and reproducing process has hitherto masked the transmission effects.
Nowadays the advent of digital recording and reproducing techniques with over 90 dB of dynamic range and below 0.01% harmonic distortion have made the problems of the analog transmission system painfully apparent. Lois D. Fielder, in the article, "Dynamic-Range Requirement for Subjectively Noise-Free Reproduction of Music", in the Journal of the Audio Engineering Society, Vol. 30, No. 7/8, 1982 July/August, pp. 504-510, states that 90-118 dB of dynamic range is needed to accurately record and reproduce classical music. Even more dynamic range is needed to accurately reproduce a live "rock and roll" concert or a jet plane flyover. In most currently used systems, the digital audio recording process usually places the analog-to-digital (A/D) converter only after the signal is processed by an analog transducer, analog preamplifier, analog transmission medium, and analog amplifier or transformer. This means that the noise, distortion, and interference of the analog transmission system will be unavoidably recorded and reproduced by the digital recording.
An analog transmission usually has a single path of conduction, such as a wire pair. Digital transmission usually needs several conductors or paths. The cost and complexity of the overall system increases with the number of transmission paths.
The signals from the transducer are often processed after transmission, for sound modification or improvement in their quality. This involves tone controls, equalizers, noise gates, anti-feedback, and other forms of processing in the time or frequency domain. These processors are physically large and power consuming, and are generally located remotely from the transducer. These processors cannot be easily controlled by someone at the transducer end, but need to be controlled locally. In a sound studio, for example, the volume and tone of a hand-held microphone cannot be changed by the microphone's user.
The resolution of existing A/D conversion techniques is limited by theoretical considerations and practical implementation to a number of bits, which is not sufficient to accurately represent the full dynamic range of an audio signal such as a live concert. High resolution A/D converters are large, power consuming, and expensive.
Also, the requirement for a power source and conduction of signals generally dictates the use of wires and cables to connect the transducer with the equipment that receives the signal. In many applications, these wires pose operational problems and can constitute electrical hazards. So called "wireless" microphones, using radio or infrared, have been used to overcome this problem; but these still operate in the analog domain and ususally impose additional noise and distortion penalties beyond those encountered in a wired system. The wireless microphones use self-cointained batteries, but these may be drained of energy and grow weak at any time, thus interrupting transmission.
The existing techniques of A/D conversion require that the analog signal be bandlimited by an input anti-aliasing filter, and be sampled by a sample-and-hold circuit prior to A/D conversion. These components require additional power and space and also degrade dynamic range and harmonic distortion performance.